The most extensive assessment of lifespan disparities between male and female mammals and birds has provided new insights
into why women generally outlive men.

The average global life expectancy for women is around 74 years, compared to 68 years for men. Various theories propose reasons for this difference, such as the tendency of younger men to suffer higher rates of accidents or conflicts, and the protection women have against dangerous mutations in sex chromosomes. However, the explanations remain incomplete.

In a search for answers from other species,
Johanna Stärk from the Max Planck Institute for Evolutionary Anthropology and her team examined data on average life expectancy across 1,176 species (528 mammals and 648 birds) observed in both zoos and in the wild.

They discovered that in 72% of mammal species, females live longer than males by an average of 12 to 13%. Conversely, in birds, males tend to outlive females in 68% of species, surviving approximately 5% longer.

Researchers posit that this trend supports the hypothesis that sex chromosomes may account for some lifespan differences.

In mammals, the presence of two X chromosomes in females grants individual genetic protection, unlike males who possess an X and a Y chromosome. This may provide women with a better safeguard against harmful mutations.

For birds, the sex determination system is reversed; females have one Z and one W chromosome while males carry two Z chromosomes. This discrepancy suggests that the gender on different chromosomes may incur different longevity costs.

“What’s particularly intriguing is when we identify exceptions,” says team member
Fernando Colcello,
also at the Max Planck Institute. “These exceptions prompt further testing of evolutionary theories regarding gender differences in lifespan.”

Diving deeper into the data, the researchers found that mating systems might influence these outcomes. In polygamous mammals, where competition is prevalent among males, those individuals usually have shorter lifespans than females.

“In the race for mating opportunities, males often invest in characteristics favored in sexual selections, such as size or ornate features,” explains
Nicole Riddle from the University of Alabama.
“These traits are costly to develop, and competition can lead to additional expenditures on aggressive behaviors.”

Such factors can result in reduced resources available for personal survival, she states.

This phenomenon is also seen in polygamous birds. “Overall, this may explain the comparatively lower male dominance in bird species,” states
Pau Carazo from the University of Valencia in Spain.

He explains that in mammals, both genetic factors and sexual selection influence shorter male lifespans, while in birds, strong sexual selection pressures on males and the varied costs for females may counterbalance the effects.

Stärk and her colleagues additionally found that those who invest more in offspring care tend to live longer, which is predominantly the case for females in mammals. In long-lived species such as humans and primates, this trait may confer an evolutionary advantage by ensuring that mothers survive until their offspring reach sexual maturity.

However, exceptions exist. “In predatory species, the opposite pattern emerges,” notes Stärk. “Females are typically larger and often take on roles in territory defense, yet they still exhibit longer lifespans,” leaving this phenomenon an ongoing enigma.

Carazo remarks that gender lifespan differences can lead to smaller populations in zoos compared to the wild. This controlled environment may also be a factor in the reduced lifespan discrepancies observed among human genders, though these differences are unlikely to vanish completely.

“Significant physiological and genetic differences between men and women remain,” explains Colcello. “The future trajectory of medical science is uncertain, but we generally do not anticipate these differences to fully eliminate.”